1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a print data generating method, and in particular, to print data generation upon performing printing by using a print head unit equipped with a plurality of print heads having overlapped printing regions.
2. Description of the Related Art
In a print head unit for use in an inkjet printing apparatus such as a printer, in which a plurality of print heads are arrayed so that nozzles of the print heads are arrayed over a certain range of a length that is longer than a length of one of the plurality of print heads, the print heads are arrayed in such a manner that respective parts of the adjacent print heads have overlapped printing regions each other. Print data for the respective parts of the adjacent print heads for printing the overlapped printing region is generated in a different manner from that of generation of print data for the other region not overlapped. That is, the print data for the overlapped printing region is subjected to a print data generating process of being distributed to each print head in a predetermined ratio (hereinafter, called a connecting process).
In the print head unit with many nozzles, such as the aforementioned print head unit structured for a plurality of print heads to be arrayed, there are many cases where any variation occurs in printing characteristics such as an ejection amount among the nozzles. Accordingly, there have been conventionally made proposals for reducing density unevenness in a print image due to such variations. U.S. Pat. No. 7,249,815 describes an example thereof where actual liquid droplet amounts of a plurality of nozzles are found and a use ratio of nozzles having a relatively large ejection liquid droplet amount and nozzles having a relatively small ejection liquid droplet amount, that is, a distribution ratio of print data, is determined based upon the actual liquid droplet amounts, thereby achieving a target average liquid droplet amount as a whole. Therefore, even if there occur variations in the printing characteristics such as an ejection amount among the nozzles, the printing in which density unevenness is reduced can be performed.
In some cases, however, in the technology described in U.S. Pat. No. 7,249,815, while the variation in the printing density is corrected, a pattern of dots printed is changed. That is, depending upon the use ratio of nozzles having a relatively large ejection liquid droplet amount and nozzles having a relative small ejection liquid droplet amount, a dot which is supposed to be originally printed by some nozzle is printed by the other nozzle based upon the above distribution, so that the printing position possibly differs. In consequence, when correction by more than a predetermined degree is made, while the density is corrected, a difference in the dot pattern is visualized, leading to degradation of image quality.
Incidentally, even if there does not occur the difference in the dot pattern by appropriately performing the distribution of the print data among nozzles correspondingly to the printing characteristics described above, an application of the aforementioned connecting process raises a problem that exclusiveness of the print data in the above distribution process is damaged. As a result, there occurs a defect or overlap of the print dots, thus generating different density unevenness.